1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy and, more particularly, to a cathode of silver vanadium oxide (SVO) admixed with a combination of polymeric binders. Importantly, the silver vanadium oxide admixture is coated to a current collector for incorporation into an electrochemical cell. The preferred binder formulation is a mixture of a halogenated polymeric binder and a butadiene rubber, most preferably polyvinylidene fluoride (PVDF) and a styrene butadiene rubber (SBR).
2. Prior Art
Halogenated polymeric materials have been used extensively as binders for electrodes in all types of nonaqueous electrochemical cells, and particularly lithium cells. For example, polyvinylidene fluoride is a material that functions well as an electrode active binder at or near ambient temperature. PVDF is generally stable in various electrochemical cell electrolytes. The addition of PVDF has been known to increase electrode flexibility as well as enable an acceptable pulse performance of the resulting electrochemical cell. However, the addition of PVDF generally provides little to no cathode material adhesion in conjunction with SVO active material. Thus, SVO cells manufactured with PVDF as the sole binder material cannot be used for direct current collector coating processes because of the material's lack of cathode material adhesion. Such an electrode produced with PVDF as a sole binder generally results in delaminated electrodes.
It is also known in the prior art to employ non-halogenated polymeric materials as binders in nonaqueous, alkali metal electrochemical cells. Exemplary is U.S. Pat. No. 5,468,571 to Asami et al. which discloses that polyimide (PI) is useful as an anode binder in lithium secondary cells. Electrodes containing such non-halogenated polymers as the sole binder material are somewhat brittle and have a tendency to crack.
U.S. Pat. No. 6,797,019 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated by reference herein, describes a process for manufacturing a cathode component prepared from a mixture of a soluble polyimide and PVDF combined with an active material such as silver vanadium oxide (SVO). As compared to the electrochemical cells disclosed in U.S. Pat. Nos. 6,001,507 and 5,571,640, this process provided an electrochemical cell with improved discharge efficiency, especially during pulsing. The mixture of polyimide and PVDF increased adhesion of the SVO active cathode material to the current collector. Furthermore, the addition of the PVDF improved flexibility of the cathode as the polyimide constituent is relatively brittle. However, the addition of the polyimide constituent requires a heat curing process. This heat curing process adds an additional manufacturing processing step that undesirably adds cost and complexity to the manufacturing process of the electrochemical cell. In addition, it has been demonstrated that under certain conditions, the heat curing process decreases the voltage output of the electrochemical cell over a period of time. This reduction in voltage output may be brought about by an undesirable reaction, which affects the chemical structure of the SVO material. Furthermore, it is believed that the heat curing process may also result in an undesirable increase in internal electrical resistance.
Therefore, there is a need for a binder formulation that provides flexibility and direct cathode material to substrate adhesion that does not result in an increase in internal electrical resistance and voltage output degradation of the electrochemical cell. As will, be described in detail hereinafter, coating a slurry of electrode active material, particularly of silver vanadium oxide, to a current collector results in improved performance characteristics in comparison to the prior art techniques. This improvement is believed to be directly attributable to the novel binder mixture.